QemuOpts: move qemu_opt_del ahead for later calling

QemuOpts: move qemu_opt_del ahead for later calling

In later patch, qemu_opt_get_del functions will be added, they will
first get the option value, then call qemu_opt_del to remove the option
from opt list. To prepare for that purpose, move qemu_opt_del ahead first.

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Leandro Dorileo <l@dorileo.org>
Signed-off-by: Chunyan Liu <cyliu@suse.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>